1. FIELD OF THE INVENTION
This invention relates to photography. In one aspect, the invention relates to a process for increasing the sensitivity of photosensitive silver halide layers especially silver halide emulsion layers and especially silver halide emulsion layers coated on a support. Products obtained in accordance with the process disclosed herein manifest increased photographic speed and reduced or substantially eliminated reciprocity law failure.
2. DESCRIPTION OF THE PRIOR ART
Various techniques have been disclosed for increasing the speed of silver halide emulsions which are employed in photograhic applications. It is well known, for example that gelatino, photographic silver halide emulsions can be sensitized chemically with a variety of chemical sensitizers. Illustrative of the chemical sensitizers which have been employed are labile sulfur compounds, selenium and tellurium compounds, certain noble metals, so-called reduction sensitizers, or combinations of chemical sensitizers. The chemical sensitizers are described, for example, in U.S. Pat. No. 1,623,499 of Sheppard et al., issued Apr. 5, 1927; U.S. Pat. No. 2,399,083 of Waller et al., issued Apr. 29, 1946; U.S. Pat. No. 3,297,446 of Dunn, issued Jan. 10, 1967 and U.S. Pat. No. 3,442,653 of Dunn, issued May 6, 1969.
Increasing the grain size of silver halides has been employed in order to provide silver halide materials which evidenced increased sensitivity. The application of vacuum treatment for photographic elements have also been employed in order to increase the speed of photographic materials. Vacuum treatment is described in Belgium Pat. No. 734,571 Aug. 14, 1969. In accordance with the Belgium patent, degassing a photographic silver halide emulsion through use of a high vacuum, such as, for example pressures of about 10.sup.-.sup.3 torr and exposing the photographic materials in the substantial absence of oxygen and moisture results in some increase in photographic sensitivity of the photographic materials.
The use of pneumatic pressure has been employed as a technique for effecting photographic sensitivities. Journal of the Optical Society of America, Vol. 34, 1944, pages 285-289, "Influence of Pneumatic Pressure on the Photographic Sensitivity" by Choong Shin-Piaw a technique is described whereby photosensitive silver halide emulsion layers are subjected to pneumatic pressure prior to exposure of such layers to actinic radiation. This reference teaches the use of pneumatic pressure applied by hydrogen, nitrogen or carbon dioxide.
It is also known from U.S. Pat. No. 3,542,557 of Illingsworth et al. that the sensitivity of silver halide emulsions can be increased if the liquid, bulk emulsions are subjected to pneumatic pressure prior to such emulsions being coated-out onto supporting surfaces. Illingsworth et al. teach that the pneumatic pressure may be supplied by the use of a wide variety of gases, including nitrogen, carbon dioxide and hydrogenm, as well as inert gases such as neon, helium, argon, krypton and xenon.
While the processes described above have succeeded in enhancing the sensitivity and hence increasing the speed of photosensitive silver halide emulsion layers, they have not succeeded in increasing the speed to the highly unexpected extent described herein. Moreover, such processes have not been successful in eliminating the problem of reciprocity law failure particularly at very low light intensities, such as those, for example, encountered in the field of astronomy, where it is often desired to photograph very faintly luminous objects which are usually far from our planet.
Accordingly there is a need for improved methods for increasing the sensitivity of photosensitive silver salt layers thereby increasing the speed of such silver salt layers and substantially reducing the reciprocity law failure.
It is an object of this invention to increase the inherent photographic speed of photographic elements. It is another object to increase the existing spectral sensitivity of photographic emulsions which have been treated with spectral sensitizers such as one or more spectral sensitizing dyes. It is yet another object of this invention to provide photographic emulsions which exhibit good high contrast. It is still another object of this invention to provide photographic emulsions which do not suffer from reciprocity law failure. It is still another object of this invention to increase the inherent and spectral sensitivity of photographic emulsions without the use of heat or hypersensitizing agents. It is still another object of the invention to increase the sensitivity of photographic emulsions which can be processed through the use of heat.